1. Field of the Invention
The present invention relates to an improvement on a focus detecting device which is adapted to be equipped on a digital camera or the like and which time-sequentially divides the photographing light beam passing through an optical system into at least two different areas, converts optical images, time-sequentially focused on image sensor means through respective areas, into image signals and detects the phase difference of the image signals thereby calculating the focus state of the optical system.
2. Related Background Art
Among the automatic focusing devices employed in the conventional camera which uses silver halide-based film of the single lens reflex type, those based on the phase difference detecting system are widely employed.
FIG. 56 is a view showing the arrangement of the optical system of a single lens reflex type, provided with a conventional focus detecting device of the phase difference detecting system, wherein a light beam 9 a emerging from a photographing lens 1 is partly reflected by a main mirror 2, consisting of a half mirror, as a light beam 9b toward a focusing screen 3 and focuses an image of the object on a matted face thereof. The photographer observes the object image on the focusing screen, through eyepiece lenses 5a, 5b and a pentagonal roof prism 4.
On the other hand, among the light beam 9a, a part 9e transmitted by the main mirror 2 is reflected by a sub mirror 6 and is guided as a light beam 9f to a focus detecting device 7, which detects, by means of the light beam 9f, the focus state (defocus amount) of the photographing lens 1 relative to a silver halide-based photographic film 8.
When the detected defocus amount is larger than a predetermined range of focus so that the photographing lens is judged as being in an out-of-focus state, an unrepresented control circuit so drives a focusing lens of the photographing lens 1 as to cancel the defocus amount, thereby achieving the focusing operation.
In the following there will be explained, with reference to FIGS. 57A to 57C, the focus detecting principle of the conventional focus detecting device.
FIG. 57A shows an in-focus state, wherein light beams 16a, 16b passing respectively through two different pupils of a photographing lens 10 are focused on a primary focal plane 14, and images of the object on such primary focal plane are refocused by secondary imaging lenses 12a, 12b on a sensor plane 13 having two line sensors for each of the refocused images. A field lens 11 is provided in the vicinity of the primary image plane of the photographing lens 10, thus efficiently guiding the light beam of a predetermined image height to the sensor plane 13 and preventing the loss in the amount of light resulting form the increase in the image height. The two light beams 16a, 16b passing through the different pupils of the photographing lens 10 are in general limited by unrepresented diaphragms positioned immediately in front of or immediately behind the secondary imaging lenses 12a, 12b, and the photographing lens 10 is not provided with a member for dividing the pupil. Assuming that xcex40 is the relative distance (phase difference) of the positions of the two images in the in-focus state, the amount of defocus in the current state and the direction thereof can be known from the difference between xcex40 and the actually given phase difference.
FIG. 57B shows a state in which the lens is focused in front by a defocus amount d1, wherein the phase difference xcex4 of the two images becomes smaller than xcex40, and the difference (xcex40xe2x88x92xcex41) increases with the increase of d1.
FIG. 57C shows a state in which the lens is focused in the back by a defocus amount d2, wherein the phase difference xcex42 of the two images becomes larger than xcex40, and the difference (xcex42xe2x88x92xcex40) increases with the increase of d1. In this manner the detection of the phase difference of the two images focused on the sensor plane 13 allows to detect the focus state of the photographing lens, or the magnitude and the direction of the defocus amount.
However, in the prior art, the single lens reflex camera becomes inevitably bulky for securing the space for the focus detecting device and becomes expensive by the cost required therefor. Also the accuracy of focusing is deteriorated as the relative positional relationship between the photographic film and the focus detecting device is varied by a change in the temperature or by a time-dependent variation in the quick return mirror.
The image taking apparatus such as a digital camera employ a solid-state image pickup device as the image sensor and can avoid the above-mentioned drawbacks by employing such solid-state image pickup device as the focus detecting sensor. A method for utilizing the solid-state image pickup device for the focus detecting sensor, for example employed in video cameras, consists of effecting the focusing operation, based on the detected contrast of the object image on the solid-state image pickup device, but such method is incapable of high-speed and highly precise focusing, as the exact defocus amount cannot be detected.
The highly precise high-speed focusing, without the above-mentioned drawbacks, can however be achieved by providing the photographing lens with pupil dividing means for rendering transmissive either one of the two different pupil areas, and detecting the relative positional difference, or the phase difference, between the object image obtained by the light beam transmitted by such transmissive pupil area and an object image obtained by the light beam transmitted by the other pupil area, thereby detecting the defocus amount.
However, in such conventional configuration, the image signal of the object image formed by the light beam transmitted by a pupil area is stored at first and then that of the object image formed by the light beam transmitted by the other pupil area is stored later, so that the two image signal storage operations are mutually different in-time.
FIGS. 58A and 58B are views showing the phase differences in the focus detecting operation respectively for a still object which is in a constant positional relationship relative to the camera and a moving object of which positional relationship varied relative to the camera, wherein L indicates an image signal corresponding to an object image formed by a light beam transmitted through a left pupil of the photographing lens, while R indicates an image signal corresponding to an object image formed by a light beam transmitted through a right pupil of the photographing lens.
In case of a still object as shown in FIG. 58A, the phase difference xcex412 between an image signal L1 stored first and an image signal R2 stored later is same as the true phase difference xcex411 corresponding to the defocus amount, so that the exact focus detection is possible.
On the other hand, in case of a moving object as shown in FIG. 58B, the object image moves on the solid-state image pickup device by xcex4m between the storage of the image signal L1 and that of the image signal R2, so that the phase difference xcex412 obtained by the image signals L1 and R2 corresponds to the sum of the true phase difference xcex411 and the moving amount xcex4m. Consequently the result of the focus detection includes an error corresponding to the moving amount xcex4m whereby the precision of the focus detection is correspondingly deteriorated.
A representative example of the change in the relative positional relationship between the camera and the object is in case of hand vibration in a hand-held photographing operation. The error in the focus detection, caused by such hand vibration, increases with the photographing lens of a longer focal length and with a longer time difference between the storage of the two image signals.
One aspect of the application is to provide a focus detecting device capable of compensating the relative movement between the object and the photographing optical system, even in case the object moves relative to the optical system in the course of the photographing operation.
One aspect of the application is to provide a focus detecting device capable of compensating the relative movement between the object and the photographing optical system, by storage, for obtaining plural images for focus detection, at least twice the image formed by the light beam transmitted by at least a pupil area, in addition to the storage of the respective images formed by the light beams transmitted by the different pupil areas.
One aspect of the application is to provide a focus detecting device capable of eliminating the error in the focus detection, resulting from an optical image moving at a constant velocity, utilizing two sets of the phase difference information obtained from three image signals.
One aspect of the application is to provide a focus detecting device capable of eliminating the error in the focus detection, resulting from an optical image moving at a constant velocity or at a constant acceleration, utilizing two sets of the phase difference information obtained from three image signals.
One aspect of the application is to provide a focus detecting device capable of eliminating the error in the focus detection, resulting from an optical image moving at a constant velocity or at a constant acceleration, utilizing at least three sets of the phase difference information obtained from at least four image signals.
One aspect of the application is to provide a focus detecting device capable of eliminating the error in the focus detection utilizing plural phase difference information, wherein a suitable number of phase difference information are selected according to the conditions such as the focal length of the photographing lens and the time interval of the storage of the image signals, thereby improving the accuracy of the focus detection and reducing the time required therefor.
Still other aspects of the application, and the features thereof, will become fully apparent from the following description of the embodiments.